Wavelength division multiplexing technologies, applying state-of-the-art approaches allow satisfying applicable requirements to the carrying capacity of FOCS. Nevertheless, the meeting of new and ever increasing requirements of communication system developers requires further improvement and development of the applied technologies. One of the directions of wavelength division multiplexing development is related to applying an approach in which carrying channel frequencies become dynamically readjusted.
The devices which are well-known and exist in a wide spectral range are laser diodes, as well as controllable optic input/output multiplexers. The optical multiplexers with tunable optic carrier channels are needed (hereinafter—controllable optic multiplexers) for connection systems with spectral compression (herein also called wavelength-division) multiplexing.
The controllable optical multiplexers may be used in their basic application as devices for the channels integration and for input to an optic line. They may be part of more complicated devices and wavelength-division multiplexing systems with dynamic functions, e.g. of multi-channel controllable input/output multiplexers.
The controllable optic multiplexers may be applied in multi-channel sensor systems, in optic analogue systems of various designations, for optic filtration and other purposes.
At present, many types of these optic multiplexers have been designed and applied. These are multiplexers in the form of multi-stage structures based on the interference filters or diffraction lattices, multiplexers designed as planar devices based on the so-called phased bundles (AWG) and finally, those closest to the present invention, multi-stage tree structures based upon asymmetric Mach-Zehnder interferometers (MZIs).
It has been known that the MZIs are characterized by small optic losses and have low polarization dependence. The MZI structure with 8÷9 stages is characterized by high selectivity and is capable to overlap the full spectral band utilized in dense wavelength division multiplexing systems.
The known traditional design of a multiplexer based on MZIs, intended for use in a fiber-optical communication system with wavelength-division multiplexing 2N channels and a frequency interval between adjacent channels Δv, is an N-stage structure of a ‘tree’ type containing 2N-n MZIs in each n-step. The multiplexer has 2N inputs, wherein one channel is passed on each input, and it has one output for a multiplexed optical signal.
Upon receipt of the channels subject to multiplexing on each input of such device, the MZIs of the first stage combine the channels into 2N-1 groups. Each group is further directed to the second stage, in which the channels are combined again, in this case into 2N-2 groups. Such process of multiplexing of the groups (and channels) proceeds with sequential passing radiation at all stages of the multiplexer. Finally, at the last stage, all channels are completely combined into one stream (an optical signal).
The optical multiplexers based on the MZIs, as well the other multiplexers above-listed, in overwhelming majority, are static, i.e. have fixed spectral characteristics, and thus they cannot be used in the FOCS with wavelength-division multiplexing, in which frequencies of the channels are dynamically adjusted.
It has been known also that, the single-stage MZIs, supplied with phase shift elements, can be key elements in the controllable optical input/output multiplexer (U.S. Pat. No. 6,795,654). It is evident that in case of proper usage, they could become also a basis for creation of controllable optical multiplexers.
The state-of-the-art optical technology provides a sufficiently wide range of MZIs, which can be used in multiplexers, including those with dynamic functions. The simplest base structure of the MZIs is an asymmetric single cascade or single-stage MZI (hereinafter—single-stage MZI).
The disadvantage of the single-stage MZIs is a non-ideal shape of spectral characteristics, which can cause cross-wise interferences (“crosstalk disturbance”) and improper isolation of the channels when used in DWDM systems. The two-stage (two-cascade) asymmetric MZIs and multi-stage (multi-cascade) asymmetric MZIs (hereinafter—two-stage and multi-stage MZIs) have superior spectral characteristics. Furthermore, the multi-stage MZIs are characterized by significantly lower induced dispersion.
It is evident that the controllable optic multiplexer which could be designed in the form of a multi-stage structure including sufficiently great number of MZIs, should be as much as possible protected against environmental influence: temperature instability, vibrations, etc. Therefore, to ensure necessary stability and reliability, the device should have large-scale integration of the used MZIs and should be compact; so the integrated-optical structure is best suited to produce such device.